Photochromic or phototropic glasses had their genesis in U.S. Pat. No. 3,208,860. Such glasses become darker, i.e., they change color, when subjected to actinic radiation, customarily ultraviolet radiation, and return to their original state when withdrawn from exposure to that radiation. That patent was addressed generally to silicate-based glasses, most preferably alkali metal aluminoborosilicate-based glasses, containing crystallites of at least one silver halide from the group of silver chloride, silver bromide, and silver iodide. The mechanism underlying the reversible darkening capability of those silver halide glasses was discussed in some detail in that patent and that explanation is incorporated herein by reference.
The most widespread commercial application for photochromic glasses has been as spectacle lenses, i.e., as prescription lenses and as non-prescription sunglasses. One of the first commercial photochromic ophthalmic lenses was marketed by Corning Glass Works, Corning, N.Y., under the trademark PHOTOGRAY.
Recently, some manufacturers of ophthalmic products have questioned whether the eye might be damaged by long term exposure to ultraviolet radiation. Particular attention has been directed to limiting transmission in the UV-B, or erythemal, zone. Such a limitation, if imposed, would present a potential problem to a photochromic lens manufacturer who considers using thinner glass with lower silver content in lens manufacture.
The thinner the lens the more lightweight it will be. Further, since silver is the single most expensive component of the glass, a reduction in the content thereof self-evidently decreases the cost of the glass. Unfortunately, however, those actions reduce the capability of the glass to absorb ultraviolet radiations in the erythemal zone. There is, of course, lack of agreement on the need to limit ultraviolet transmission. Nevertheless, to the extent that a glass lens of reduced transmission is deemed necessary or desirable, the present invention makes provision therefor in a thin, low-silver-content blank.
U.S. Pat. No. 4,407,966, issued Oct. 4, 1983 in the names of David J. Kerko and David L. Morse, discloses the utility of CeO.sub.2 to reduce the transmittance at wavelengths in the near ultraviolet region of the radiation spectrum. However, it was also observed that the addition of any significant level of CeO.sub.2 substantially degraded photochromic performance.